Can I use an equation to change variable values while keeping a separate variable at constant value?

Question

I want to make a pendulum. I have a line with a ball fixed to both ends of the line. Ball1 is fixed in place. I want ball2 to "fall" (move down the screen) and I want it to "swing" on the line. AKA I want the ball to move down and I want the line to stay at a constant length forcing the ball to arc and "swing" on the line.

`
Part part1;
Part part2;
Muscle muscle1;
float gravity = 10;

void setup() {
  ...
}

void draw() {
  background(255);
  part1.drawpart();
  part2.drawpart();
  muscle1.drawmuscle(part1, part2);
  part2.movepart();
}

class Muscle{
  float leftx;
  float lefty;
  float rightx;
  float righty;
  float size = 100;
  int musclecolor;
  
  ...
  
  void drawmuscle(Part obj1, Part obj2) {
    strokeWeight(5);
    line(obj1.x, obj1.y, obj2.x, obj2.y);
    size = sqrt(sq(obj1.x - obj2.x) + sq(obj1.y - obj2.y)); 

^^ This is the equation to keep the line the same length. Will this equation work if placed in the right location? Or is this method just not possible?

  }
}

class Part{
  float x;
  float y;
  float size;
  int partcolor;
  
  ...
  void drawpart() {
    ...
    ellipse(x, y, size, size);
  }
  
  void movepart() {
   part2.y += gravity;
  }
}